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# Copyright 2015 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ==============================================================================
"""Gradients for operators defined in linalg_ops.py.
Useful reference for derivative formulas is
An extended collection of matrix derivative results for forward and reverse
mode algorithmic differentiation by Mike Giles:
http://eprints.maths.ox.ac.uk/1079/1/NA-08-01.pdf
A detailed derivation of formulas for backpropagating through spectral layers
(SVD and Eig) by Ionescu, Vantzos & Sminchisescu:
https://arxiv.org/pdf/1509.07838v4.pdf
"""
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
from tensorflow.python.framework import ops
from tensorflow.python.ops import array_ops
from tensorflow.python.ops import control_flow_ops
from tensorflow.python.ops import linalg_ops
from tensorflow.python.ops import math_ops
@ops.RegisterGradient("MatrixInverse")
def _MatrixInverseGrad(op, grad):
"""Gradient for MatrixInverse."""
ainv = op.outputs[0]
return -math_ops.matmul(
ainv, math_ops.matmul(
grad, ainv, adjoint_b=True), adjoint_a=True)
@ops.RegisterGradient("MatrixDeterminant")
def _MatrixDeterminantGrad(op, grad):
"""Gradient for MatrixDeterminant."""
a = op.inputs[0]
c = op.outputs[0]
a_adj_inv = linalg_ops.matrix_inverse(a, adjoint=True)
multipliers = array_ops.reshape(
grad * c, array_ops.concat([array_ops.shape(c), [1, 1]], 0))
return multipliers * a_adj_inv
@ops.RegisterGradient("Cholesky")
def _CholeskyGrad(op, grad):
"""Gradient for Cholesky."""
return linalg_ops.cholesky_grad(op.outputs[0], grad)
@ops.RegisterGradient("MatrixSolve")
def _MatrixSolveGrad(op, grad):
"""Gradient for MatrixSolve."""
a = op.inputs[0]
adjoint_a = op.get_attr("adjoint")
c = op.outputs[0]
grad_b = linalg_ops.matrix_solve(a, grad, adjoint=not adjoint_a)
if adjoint_a:
grad_a = -math_ops.matmul(c, grad_b, adjoint_b=True)
else:
grad_a = -math_ops.matmul(grad_b, c, adjoint_b=True)
return (grad_a, grad_b)
@ops.RegisterGradient("MatrixSolveLs")
def _MatrixSolveLsGrad(op, grad):
"""Gradients for MatrixSolveLs."""
# TODO(rmlarsen): The implementation could be more efficient:
# a) Output the Cholesky factorization from forward op instead of
# recomputing it here.
# b) Implement a symmetric rank-k update op instead of computing
# x*z + transpose(x*z). This pattern occurs other places in TensorFlow.
def _overdetermined(op, grad):
"""Gradients for the overdetermined case of MatrixSolveLs.
This is the backprop for the solution to the normal equations of the first
kind:
X = F(A, B) = (A^T * A + lambda * I)^{-1} * A^T * B
which solve the least squares problem
min ||A * X - B||_F^2 + lambda ||X||_F^2.
"""
a = op.inputs[0]
b = op.inputs[1]
l2_regularizer = math_ops.cast(op.inputs[2], a.dtype.base_dtype)
x = op.outputs[0]
a_shape = array_ops.shape(a)
batch_shape = a_shape[:-2]
n = a_shape[-1]
identity = linalg_ops.eye(n, batch_shape=batch_shape, dtype=a.dtype)
gramian = math_ops.matmul(a, a, adjoint_a=True) + l2_regularizer * identity
chol = linalg_ops.cholesky(gramian)
# Temporary z = (A^T * A + lambda * I)^{-1} * grad.
z = linalg_ops.cholesky_solve(chol, grad)
xzt = math_ops.matmul(x, z, adjoint_b=True)
zx_sym = xzt + array_ops.matrix_transpose(xzt)
grad_a = -math_ops.matmul(a, zx_sym) + math_ops.matmul(b, z, adjoint_b=True)
grad_b = math_ops.matmul(a, z)
return (grad_a, grad_b, None)
def _underdetermined(op, grad):
"""Gradients for the underdetermined case of MatrixSolveLs.
This is the backprop for the solution to the normal equations of the second
kind:
X = F(A, B) = A * (A*A^T + lambda*I)^{-1} * B
that (for lambda=0) solve the least squares problem
min ||X||_F subject to A*X = B.
"""
a = op.inputs[0]
b = op.inputs[1]
l2_regularizer = math_ops.cast(op.inputs[2], a.dtype.base_dtype)
a_shape = array_ops.shape(a)
batch_shape = a_shape[:-2]
m = a_shape[-2]
identity = linalg_ops.eye(m, batch_shape=batch_shape, dtype=a.dtype)
gramian = math_ops.matmul(a, a, adjoint_b=True) + l2_regularizer * identity
chol = linalg_ops.cholesky(gramian)
grad_b = linalg_ops.cholesky_solve(chol, math_ops.matmul(a, grad))
# Temporary tmp = (A * A^T + lambda * I)^{-1} * B.
tmp = linalg_ops.cholesky_solve(chol, b)
a1 = math_ops.matmul(tmp, a, adjoint_a=True)
a1 = -math_ops.matmul(grad_b, a1)
a2 = grad - math_ops.matmul(a, grad_b, adjoint_a=True)
a2 = math_ops.matmul(tmp, a2, adjoint_b=True)
grad_a = a1 + a2
return (grad_a, grad_b, None)
fast = op.get_attr("fast")
if fast is False:
raise ValueError("Gradient not defined for fast=False")
matrix_shape = op.inputs[0].get_shape()[-2:]
if matrix_shape.is_fully_defined():
if matrix_shape[-2] >= matrix_shape[-1]:
return _overdetermined(op, grad)
else:
return _underdetermined(op, grad)
else:
# We have to defer determining the shape to runtime and use
# conditional execution of the appropriate graph.
matrix_shape = array_ops.shape(op.inputs[0])[-2:]
return control_flow_ops.cond(matrix_shape[-2] >= matrix_shape[-1],
lambda: _overdetermined(op, grad),
lambda: _underdetermined(op, grad))
@ops.RegisterGradient("MatrixTriangularSolve")
def _MatrixTriangularSolveGrad(op, grad):
"""Gradient for MatrixTriangularSolve."""
a = op.inputs[0]
adjoint_a = op.get_attr("adjoint")
lower_a = op.get_attr("lower")
c = op.outputs[0]
grad_b = linalg_ops.matrix_triangular_solve(
a, grad, lower=lower_a, adjoint=not adjoint_a)
if adjoint_a:
grad_a = -math_ops.matmul(c, grad_b, adjoint_b=True)
else:
grad_a = -math_ops.matmul(grad_b, c, adjoint_b=True)
if lower_a:
grad_a = array_ops.matrix_band_part(grad_a, -1, 0)
else:
grad_a = array_ops.matrix_band_part(grad_a, 0, -1)
return (grad_a, grad_b)
@ops.RegisterGradient("SelfAdjointEigV2")
def _SelfAdjointEigV2Grad(op, grad_e, grad_v):
"""Gradient for SelfAdjointEigV2."""
e = op.outputs[0]
v = op.outputs[1]
# a = op.inputs[0], which satisfies
# a[...,:,:] * v[...,:,i] = e[...,i] * v[...,i]
with ops.control_dependencies([grad_e.op, grad_v.op]):
if grad_v is not None:
# Construct the matrix f(i,j) = (i != j ? 1 / (e_i - e_j) : 0).
# Notice that because of the term involving f, the gradient becomes
# infinite (or NaN in practice) when eigenvalues are not unique.
# Mathematically this should not be surprising, since for (k-fold)
# degenerate eigenvalues, the corresponding eigenvectors are only defined
# up to arbitrary rotation in a (k-dimensional) subspace.
f = array_ops.matrix_set_diag(
math_ops.reciprocal(
array_ops.expand_dims(e, -2) - array_ops.expand_dims(e, -1)),
array_ops.zeros_like(e))
grad_a = math_ops.matmul(
v,
math_ops.matmul(
array_ops.matrix_diag(grad_e) + f * math_ops.matmul(
v, grad_v, adjoint_a=True),
v,
adjoint_b=True))
else:
grad_a = math_ops.matmul(
v, math_ops.matmul(
array_ops.matrix_diag(grad_e), v, adjoint_b=True))
# The forward op only depends on the lower triangular part of a, so here we
# symmetrize and take the lower triangle
grad_a = array_ops.matrix_band_part(
grad_a + array_ops.matrix_transpose(grad_a), -1, 0)
grad_a = array_ops.matrix_set_diag(grad_a,
0.5 * array_ops.matrix_diag_part(grad_a))
return grad_a